P.P.P.M. Lerou

Fabrication of a micro cryogenic cold stage using MEMS-technology

This paper describes the design and production process of a variety of reliable micro cryogenic coolers. The different cold stages are based on an optimized design found during a study which was done to maximize the cold-stage effectiveness. Typical cold-stage dimensions are 30 × 2 × 0.5 mm with an expected net cooling power varying from 10 mW to 20 mW at a tip temperature of 96 K. A cold stage consists of a stack of three fusion bonded D263T glass wafers. The production process has 7 lithography steps and roughly 100 process steps. In order to determine the maximum bend, shear and bond stresses inside a 175 µm thick D263T glass wafer, several pressure tests were performed.

Characterization of micromachined cryogenic coolers

Micro cryogenic coolers can be used to cool small circuitry and improve their performance. The authors present a variety of micro coolers which are fabricated using MEMS technology production processes only. The typical dimension of a micro cold stage is 30 × 2.2 × 0.5 mm. It cools down to 96 K, applying Joule–Thomson expansion in a 300 nm high flow restriction and has a cooling power ranging from 10 mW to 25 mW. This paper discusses the operation of the micro cold stage and the characterization measurements done.

Insight into clogging of micromachined cryogenic coolers

Cryogenic microcoolers can be used to cool small electronic devices to improve their performance. The authors present a micro-cold-stage of only 0.05cm3 that cools to 96K, applying Joule-Thomson expansion in a 300nm high flow restriction. Critical in such a microcooler is the deposition of water molecules that migrate to the restriction and block the flow. Because the microcooler is made of glass the authors had the unique opportunity to monitor this phenomenon and combine this visualization with experimental data. This provides significant insight in the way this clogging develops and opens possibilities to realize stable operation.

Micromachined cryogenic cooler for cooling electronic devices down to 30 K

Cryogenic temperatures are required for improving the performance of electronic devices and for operating superconducting sensors and circuits. The broad implementation of cooling these devices has long been constrained by the availability of reliable and low cost cryocoolers. After the successful development of single-stage micromachined coolers able to cool to 100 K, we now present a micromachined two-stage microcooler that cools down to 30 K from an ambient temperature of 295 K. The first stage of the microcooler operates at about 94 K with nitrogen gas and pre-cools the second stage operating with hydrogen gas. The microcooler is made from just three glass wafers and operates with modest high-pressure gases and without moving parts facilitating high yield fabrication of these microcoolers. We have successfully cooled a YBCO film through its superconducting transition state to demonstrate a load on the microcooler at cryogenic temperatures. This work could expedite the application of superconducting and electronic sensors and detectors among others in medical and space applications

Micromachined Joule-Thomson coolers

A MEMS-based Joule-Thomson cold stage was designed and prototypes were realized and tested. The cold stage consists of a stack of three glass wafers. In the top wafer, a high-pressure channel is etched that ends in a flow restriction with a height of typically 300 nm. An evaporator volume crosses the center wafer into the bottom wafer. This bottom wafer contains the low-pressure channel thus forming a counter-flow heat exchanger. A design aiming at a net cooling power of 10 mW at 96 K and operating with nitrogen as the working fluid was optimized based on the minimization of entropy production. A batch of prototype coolers ranging from 20 to 40 mm was made for a flow of typically 1mgCs-1 at a high pressure of 80 bar and a low pressure of 6 bar. The design and fabrication of the coolers will be discussed along with experimental results. A specific issue that will be addressed is the clogging of the restriction due to the deposition of ice crystals. Furthermore, introductory experiments with multistage microcoolers will be discussed.

Micromachined cryogenic coolers for cooling low-temperature detectors and electronics

Vibration-free miniature cryogenic coolers are relevant to a wide variety of applications, including cooling of detectors in space missions, low-noise amplifiers and superconducting electronics. For these applications, the cryogenic system (cooler plus interface) should be small, low-cost, low-interference and above all very reliable (long-life). A cold stage based on micro-electro-mechanical systems technology was designed and prototypes were realized and tested. This cooler operates on basis of the Joule-Thomson effect. A design aiming at a net cooling power of 10 mW at 96 K and operating with nitrogen as the working fluid was optimized and measures 28 mm times 2.2 mm times 0.8 mm. It operates with a nitrogen flow of 1 mg/s at a high pressure of 80 bar and a low pressure of 6 bar. The design and fabrication of the coolers is discussed along with experimental results.

Characterization of a two-stage 30 K Joule-Thomson microcooler

Micromachined cryocoolers are attractive tools for cooling electronic chips and devices to cryogenic temperatures. A two-stage 30 K microcooler operating with nitrogen and hydrogen gas is fabricated using micromachining technology. The nitrogen and hydrogen stages cool down to about 94 and 30 K, respectively, using Joule–Thomson expansion in a restriction with a height of 1.10 μm. The nitrogen stage is typically operated between 1.1 bar at the low-pressure side and 85.1 bar at the high-pressure side. The hydrogen stage has a low pressure of 5.7 bar, whereas the high pressure is varied between 45.5 and 60.4 bar. In changing the pressure settings, the cooling power can more or less be exchanged between the two stages. These typically range from 21 to 84 mW at 95 K at the nitrogen stage, corresponding to 30 to 5 mW at 31–32 K at the hydrogen stage. This paper discusses the characterization of this two-stage microcooler. Experimental results on cool down and cooling power are compared to dynamic modeling predictions

Micromachined Joule-Thomson coolers for cooling low-temperature detectors and electronics

The performance of electronic devices can often be improved by lowering the operating temperature resulting in lower noise and larger speed. Also, new phenomena can be applied at low temperatures, as for instance superconductivity. In order to fully exploit lowtemperature electronic devices, the cryogenic system (cooler plus interface) should be ‘invisible’ to the user. It should be small, low-cost, low-interference, and above all very reliable (long-life). The realization of cryogenic systems fulfilling these requirements is the topic of research of the Cooling and Instrumentation group at the University of Twente. A MEMS-based cold stage was designed and prototypes were realized and tested. The cooler operates on basis of the Joule-Thomson effect. Here, a high-pressure gas expands adiabatically over a flow restriction and thus cools and liquefies. Heat from the environment (e.g., an optical detector) can be absorbed in the evaporation of the liquid. The evaporated working fluid returns to the low-pressure side of the system via a counter-flow heat exchanger. In passing this heat exchanger, it takes up heat from the incoming high-pressure gas that thus is precooled on its way to the restriction. The cold stage consists of a stack of three glass wafers. In the top wafer, a high-pressure channel is etched that ends in a flow restriction with a height of typically 300 nm. An evaporator volume crosses the center wafer into the bottom wafer. This bottom wafer contains the lowpressure channel thus forming a counter-flow heat exchanger. A design aiming at a net cooling power of 10 mW at 96 K and operating with nitrogen as the working fluid was optimized based on the minimization of entropy production. The optimum cold finger measures 28 mm x 2.2 mm x 0.8 mm operating with a nitrogen flow of 1 mg/s at a high pressure of 80 bar and a low pressure of 6 bar. The design and fabrication of the coolers will be discussed along with experimental results.

Joule-Thomson microcooling developments at University of Twente

The development of Joule-Thomson microcoolers has been an on-going and successful research project at the University of Twente for many years. The aim of the research is to develop small and fully integrated cryogenic cooling systems for cooling small electronic devices such as pre-amplifiers and infrared sensors, in order to improve their performance. In the foregoing years, we have successfully developed single-stage microcoolers (typically cooling to 100 K) and two-stage microcoolers (typically 30 K) using standard micromachining technologies. In the present paper, we emphatically discuss recent developments in the Twente microcooling project among which microcoolers with a double expansion of the high pressure flow (reducing the 100 K to 83 K operating temperature), microcoolers operating with hydrocarbon gas mixtures, and microcoolers with an ejector, the three new developments aiming at lower cold end temperatures, lower operating pressure ratios and/or higher efficiency. Besides, utilization of microcoolers for cooling electronics and clogging phenomenon in microcoolers will also be introduced.